We discuss the uncertainty related to the amount of unwanted $B\to K_0^\ast (K\pi)\ell^+\ell^-$ events in the sample of $B\to K^\ast (K\pi)\ell^+\ell^-$. Those events can increase the measured differential decay rate by up to 10% in the low $q^2$ region, and can be a source of non-negligible uncertainty in the full angular distribution of the $B\to K^\ast (K\pi)\ell^+\ell^-$ decay. Although the transverse asymmetries should be unaffected by the presence of the $S$-wave $K\pi$ pairs, coming from the scalar $K_0^\ast$ meson, we show that in practice their normalization might be sensitive to those events and could entail a sizable uncertainty in transverse asymmetries around $q^2=2 GeV^2$. For other $q^2$'s that error is under about 10%.
Categories: hep-ph hep-ex